1. Field of the Invention
The present invention relates to a detachable connection structure for connecting optical fibers and an endoscope system using this connection structure, and more particularly relates to a connection structure of optical fibers suitable for transmitting short-wavelength, high-power laser light and the endoscope system using this connection structure.
2. Description of the Related Art
A physical contact connection (hereinafter referred to as the PC connection) is a generally used method for connecting optical fibers, in which ends of the optical fibers are tightly made into contact with each other. Each optical fiber used in the PC connection is inserted through and fixed in a cylindrical ferrule, and an end surface of the optical fiber is polished into a convex spherical surface together with an end surface of the ferrule. The ferrules with the optical fibers are inserted into opposite ends of a cylindrical sleeve, and the end surfaces of the ferrules are made into contact with each other inside the sleeve. Owing to this, the optical fibers exposed from the end surfaces of the ferrules are directly and tightly made into contact with each other.
Endoscopes are widely used for examining the inside of human bodies. The endoscope has an insertion section which is inserted into the body, and an operation section for controlling directions of a tip of the insertion section. Through the insertion section, a light guide for sending illumination light is inserted. This light guide is a fiber bundle which is composed of a plurality of optical fibers tied together. An end of the light guide is connected to a lighting window provided at a distal end of the insertion section. The other end of the light guide is connected to a light source unit via a connector using the PC connection.
Since the connector of the endoscope is frequently attached and detached, dust or the like tend to be adhered to the end of the optical fiber. In addition, the end of the optical fiber may be damaged and has flaws due to the impact of the attachment or detachment of the connector. If the end of the optical fiber gets dusty or flawed, a connection loss increases. In addition, if the optical fiber end has high optical power density, the dust adhered or the flaws thereon may catch fire, and the end of the optical fiber and the end of the ferrule may be burnt. Otherwise, the fire may spread over the optical fiber by a fiber fuse phenomenon.
In order to prevent the burn of the optical fiber due to the adhered dust or the like, there is known an optical fiber transmission path in which the optical power density is lowered at the connection part of the optical fibers. According to U.S. Pat. Nos. 7,333,702 (corresponding to Japanese Patent Laid-Open Publication No. 2005-077549) and 6,542,665 (corresponding to Japanese Patent Laid-Open Publication No. 2002-350666), for example, a graded-index fiber that functions as a collimator lens is connected to an end of a single-mode fiber by fusion splicing, so that a mode field diameter is expanded.
By the way, it is considered to use a laser light source unit as alight source of the endoscope. In the laser light source unit, laser light with short wavelength and high power is guided through the light guide to a phosphor disposed near the lighting window, and the phosphor is excited by the laser light to emanate illumination light. In the light guide used in the laser light source unit, a single strand of multi-mode fiber having a large core diameter of, for example, at least 100 μm is used instead of the optical fiber bundle. Owing to the use of the laser light source for the endoscope, a diameter of the insertion section of the endoscope can be reduced.
It is known that when the optical fiber transmits short-wavelength, high-power laser light, a phenomenon called dust collection effect occurs at a light transmission part where the optical power density is high. The dust collection effect is a phenomenon in which the laser light photochemically reacts with vaporized organic substances and forms other substances, and the formed substances are deposited. Since the multi-mode fiber has a lager core diameter than a general single-mode fiber, it is difficult to precisely contact the end surfaces of the cores of the multi-mode fibers with each other by the PC connection. As a result, apart of the end surface of the core with high optical power density is exposed to air, which causes the dust collection effect. The occurrence of the dust collection effect in the optical fiber increases the connection loss, like the case where dust adheres to the end of the optical fiber, and may causes the burn of the optical fiber or the fiber fuse phenomenon.
When the connected optical fibers transmit the short-wavelength, high-power laser light, there is a case where oxide (such as quartz and SIO2) contained in the optical fibers reacts with the laser light, and the ends of the optical fibers adhere to each other in the PC connection section. It is known that this adhesion phenomenon is likely to occur when the ends of the optical fibers or the ends of the ferrules are connected by the PC connection after UV cleaning. If the optical fibers adhere to each other in the connector of the endoscope, the ends of the optical fibers will break upon detaching the connector from the laser light source unit. If the ends of the optical fibers are broken, they need to be re-polished with the ferrules, which is a big repair.
Such dust collection effect and adhesion phenomenon can be prevented by lowering the optical power density at the PC connection section with use of the graded-index fibers like the devices disclosed in the U.S. Pat. Nos. 7,333,702 and 6,542,665. However, since these devices use the single-mode fibers and assume to transmit long-wavelength light for communications, the type and the core diameter of the optical fibers being used are different from those of the laser light source unit, and it is difficult to simply replace the light with the laser light.